1. Field of the Invention
The present invention relates to intercellular adhesion molecules such as ICAM-1 which are involved in the process through which populations of lymphocytes recognize and adhere to cellular substrates so that they may migrate to sites of inflammation and interact with cells during inflammatory reactions. The present invention additionally relates to ligand molecules capable of binding to such intercellular adhesion molecules, to a screening assay for these ligands, and to uses for the intercellular adhesion molecule, the ligand molecules, and the screening assay.
2. Description of the Related Art
Leukocytes must be able to attach to cellular substrates in order to properly defend the host against foreign invaders such as bacteria or viruses. An excellent review of the defense system is provided by Eisen, H. W., (In: Microbiology, 3rd Ed., Harper & Row, Philadelphia, Pa. (1980), pp. 290-295 and 381-418). They must be able to attach to endothelial cells so that they can migrate from circulation to sites of ongoing inflammation. Furthermore, they must attach to antigen-presenting cells so that a normal specific immune response can occur, and finally, they must attach to appropriate target cells so that lysis of vitally-infected or tumor cells can occur.
Recently, leukocyte surface molecules involved in mediating such attachments were identified using hybridoma technology. Briefly, monoclonal antibodies directed against human T-cells (Davignon, D. et al., Proc. Natl. Acad. Sci. USA 78:4535-4539 (1981)) and mouse spleen cells (Springer, T. et al. Eur. J. Immunol. 9:301-306 (1979)) were identified which bound to leukocyte surfaces and inhibited the attachment related functions described above (Springer, T. et al., Fed. Proc. 44:2660-2663 (1985)). The molecules identified by those antibodies were called Mac-1 and Lymphocyte Function-associated Antigen-1 (LFA-1). Mac-1 is a heterodimer found on macrophages, granulocytes and large granular lymphocytes. LFA-1 is a heterodimer found on most lymphocytes (Springer, T. A., et al. Immunol. Rev. 68:111-135 (1982)). These two molecules, plus a third molecule, p150,95 (which has a tissue distribution similar to Mac-1) play a role in cellular adhesion (Keizer, G. et al., Eur. J. Immunol. 15:1142-1147 (1985)).
The above-described leukocyte molecules were found to be members of a related family of glycoproteins (Sanchez-Madrid, F. et al., J. Exper. Med. 58:1785-1803 (1983); Keizer, G. D. et al., Eur. J. Immunol. 15:1142-1147 (1985)). This glycoprotein family is composed of heterodimers having one alpha chain and one beta chain. Although the alpha chain of each of the antigens differed from one another, the beta chain was found to be highly conserved (Sanchez-Madrid, F. et al., J. Exper. Med. 158:1785-1803 (1983)). The beta chain of the glycoprotein family (sometimes referred to as "CD18") was found to have a molecular weight of 95 kd whereas the alpha chains were found to vary from 150 kd to 180 kd (Springer, T., Fed. Proc. 44:2660-2663 (1985)). Although the alpha subunits of the membrane proteins do not share the extensive homology shared by the beta subunits, close analysis of the alpha subunits of the glycoproteins has revealed that there are substantial similarities between them. Reviews of the similarities between the alpha and beta subunits of the LFA-1 related glycoproteins are provided by Sanchez-Madrid, F. et al., (J. Exper. Med. 158:586-602 (1983); J. Exper. Med. 158:1785-1803 (1983)).
A group of individuals has been identified who are unable to express normal amounts of any member of this adhesion protein family on their leukocyte cell surface (Anderson, D. C., et al., Fed. Proc. 44:2671-2677 (1985); Anderson, D. C., et al., J. Infect. Dis. 152:668-689 (1985)). Lymphocytes from these patients displayed in vitro defects similar to normal counterparts whose LFA-1 family of molecules had been antagonized by antibodies. Furthermore, these individuals were unable to mount a normal immune response due to an inability of their cells to adhere to cellular substrates (Anderson, D. C., et al., Fed. Proc. 44:2671-2677 (1985); Anderson, D. C., et al., J. Infect. Dis. 152:668-689 (1985)). These data show that immune reactions are mitigated when lymphocytes are unable to adhere in a normal fashion due to the lack of functional adhesion molecules of the LFA-1 family. Thus, in summary, the ability of lymphocytes to maintain the health and viability of an animal requires that they be capable of adhering to other cells (such as endothelial cells). This adherence has been found to require cell-cell contacts which involve specific receptor molecules present on the cell surface of the lymphocytes. These receptors enable a lymphocyte to adhere to other lymphocytes or to endothelial, and other non-vascular cells. The cell surface receptor molecules have been found to be highly related to one another. Humans whose lymphocytes lack these cell surface receptor molecules exhibit chronic and recurring infections, as well as other clinical symptoms including defective antibody responses.
Since lymphocyte adhesion is involved in the process through which foreign tissue is identified and rejected, an understanding of this process is of significant value in the fields of organ transplantation, tissue grafting, allergy and oncology.